How to Size Ductless Air Conditioners

Ductless air conditioners, or mini-split AC units, use similar technology to that of standard central heat and air systems. The principal difference is that refrigerant is piped directly into fan units inside the room through small pipes, making duct work unnecessary. Sizing a ductless system requires measuring the square footage of a room and converting the measurement to British thermaul units (BTUs), a measure of energy. A large room needs a higher-BTU system for proper cooling than a smaller one.

Window units use similar technology as ductless systems.
  1. Measure the length and width of all rooms that you will connect to the ductless system. Multiply the two measurements to find the area of each room. Do not include closets when calculating the area of a room. If the room is not perfectly rectangular, measure the length and width of any alcoves separately, multiply these measurements and add them to the first measurement.

  2. Convert the square footage to total BTUs. Rooms with an area of 100 to 150 square feet call for 5,000 BTUs. Rooms of 150 to 250 square feet call for 6,000 BTUs. Rooms of 250 to 350 square feet need 7,000 BTUs. Multiply rooms larger than 350 square feet by 25 to determine the approximate BTUs for the room. Add 4,000 BTUs for kitchens and 1,000 BTUs for bathrooms.

  3. Add the total BTUs to determine the whole house BTUs. Split-systems are mostly ranked by total BTUs. Some units are ranked by tons. Divide the total BTUs by 12,000 to calculate the tonnage for the house.

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